This invention generally relates to a process for assembling electrochemical cells having a frangible electrode and, more particularly, to cells having a current collector assembly with a shield secured thereto which limits movement of the frangible electrode.
Cylindrically shaped batteries are suitable for use by consumers in a wide variety of devices such as flashlights, radios and cameras. Batteries used in these devices typically employ a cylindrical metal container to house two electrodes, a separator, a quantity of electrolyte and a closure assembly that includes a current collector. Typical electrode materials include manganese dioxide as the cathode and zinc as the anode. The zinc is commonly employed in particulate form suspended in a gel. An aqueous solution of potassium hydroxide is a common electrolyte. A separator, conventionally formed from one or more strips of paper, is positioned between the electrodes. The electrolyte is readily absorbed by the separator and gelling medium.
One of the issues that battery manufacturers must address is the requirement that direct contact between the anode and cathode within the battery be prevented. If the anode and cathode are allowed to physically contact one another, a chemical reaction takes place and the useful electrochemical capacity of the battery is reduced. The function of the separator is to prevent direct contact between the anode and cathode while allowing for ionic conductivity therebetween.
Small cylindrical batteries must be manufactured to withstand the physical rigors associated with the manufacturing and distribution processes as well as the handling of batteries by consumers. In particular, batteries must be able to withstand being accidentally dropped by consumers. In batteries with a frangible electrode, such as the gelled anode used in many cylindrical alkaline batteries, dropping the battery may cause a small portion of the anode to fragment and thereby break free from the rest of the anode. The fragmented anode must be prevented from coming into direct contact with the cathode. Similarly, if the cathode is hard and susceptible to fragmentation when the cell is dropped, small fragments of the cathode may become separated from the body of the cathode and need to be contained. As shown in FIG. 1, many conventional cell constructions have addressed this problem by using an clastomeric seal 78 that includes a V-shaped leg 80 that projects toward the interior of the cell and contacts the top of the coiled separator 20 thereby forming a barrier that prevents anode fragments 64 from contacting cathode 54. However, the V-shaped projections on the conventional seal designs occupy a portion of the cell's internal volume which could be better used to hold an additional quantity of the cell's electrochemically active materials. Consequently, many cell designs have been proposed that utilize low volume seals which do not cooperate with the separator to form a barrier that isolates the anode from the cathode. Unfortunately, eliminating the portion of the seal that helps to contain the anode has led to an increased level of internal electrical shorting between the anode and cathode when cells are dropped by consumers. The internal shorting problem is due to the freestanding portion of the separator, located above the anode/cathode interface, losing its stiffness when it absorbs some of the cell's electrolyte and then collapsing away from the low volume seal so that an unobstructed path is created between the anode and the cathode. As shown in FIG. 2, the collapsed portion 46 of the separator 20 has allowed a fragment 64 of anode 66 to contact cathode 54.
One solution to the problem of preventing internal electrical short circuits caused by fragmented electrodes in cylindrical alkaline batteries is disclosed in Japanese Kokai Patent Application No. 7 [1995]-134977. In one embodiment, this reference discloses applying an adhesive to a portion of the separator where the separator and sealing gasket contact one another. The adhesive secures the separator to the gasket so that small portions of the electrode that break free when the battery is dropped will not be able to contact the opposing electrode and cause an internal short circuit. One disadvantage with this approach is that the application of the adhesive to the edge of the separator in a large scale commercial manufacturing operation would slow down the production process thereby increasing the cost of the battery.
Disclosed in U.S. Pat. No. 3,056,849 is a cell construction with a washer placed on top of an out-turned edge of the separator that overlays the depolarizer and thus prevents the anode slurry from contacting the depolarizer cylinder. One disadvantage of this cell construction is that the washer effectively limits the height of the depolarizer and thus reduces the discharge capacity of the cell.
U.S. Pat. No. 3,756,859 discloses a process for assembling an annular disc on top of a depolarizer mass body. The body is covered at its upper end with the annular disk and then a carbon rod is inserted through the disk and into the depolarizer body. This process discloses the placement of the disk and carbon rod in two separate steps.
U.S. Pat. No. 3,888,700 discloses a cell assembly process that includes a plastic or paper compression washer and a carbon pencil which serves as a current collector. The washer has a hole therein to receive the carbon pencil. The disclosed process inserts the cathode mix into the container and then the carbon pencil is driven into the cathode mix. The washer is then placed on top of the cathode mix so that the carbon pencil aligns with the hole in the washer. This process also discloses the placement of the washer and carbon pencil in two separate steps.
There exists a need for a process that provides a cell with a volume efficient electrode containment shield that can be accurately and economically located within the cell and will prevent undesirable movement of a frangible electrode's fragments so that the cell does not experience an internal electrical short circuit when the cell is dropped. The shield should occupy a minimum amount of the cell's internal volume and should be compatible with low volume seal bodies that do not provide structural support to the cell's separator.